1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image is transferred onto a sheet with use of an intermediate transferring belt.
2. Description of the Related Art
A conventional technology related to this kind of image forming apparatus has been a technology disclosed in, for example, Japanese Patent Unexamined Publication H11-288188 (referred to as a patent document 1, hereinafter). The patent document 1 discloses image forming of primarily transferring a toner image onto an intermediate transferring belt and then secondarily transferring the toner image onto a sheet. According to the patent document 1, an electrostatic latent image is formed on a surface of a photoconductive member based on image data, and the latent image is developed by a multi-color developing device for each color and primarily transferred to the intermediate transferring belt. Then, the toner images of all colors are synthesized into one color toner image, and then the color toner image is secondarily transferred from the intermediate transferring belt onto a sheet.
Further, according to the patent document 1, there is provided a guide plate at an upstream position of the transferring roller for guiding the sheet along the surface of the intermediate transferring belt. A sheet sent from a pair of registration rollers is guided by the guide plate and conveyed in a direction along the surface of the intermediate transferring belt. The guide plate ends at an upstream position upstream of the transferring roller, and the sheet is taken over by a guide of another member (a member which is integrally formed with a holder of the transferring roller) from the end (downstream) of the guide plate. Then, the sheet is guided by the guide and sent to a nip portion between the intermediate transferring belt and the transferring roller, and then the toner image is secondarily transferred onto the sheet while the sheet passes through the nip portion.
The intermediate transferring belt used in the above-described patent document 1 is normally driven (rotationally running) in a state where a tension is given to some extent. Therefore, it is likely that an oscillation (movement) with a flexure in a thickness direction on a rotational path of the belt. Such flexural oscillation of the intermediate transferring belt changes a distance between the sheet and the intermediate transferring belt in an area immediately before a nip portion between the transferring roller and the intermediate transferring belt (an upstream position in the conveying direction).
On the other hand, in the case where a sheet is conveyed in a direction along a surface of the intermediate transferring belt like the patent document 1, the sheet and the intermediate transferring belt are extremely close to each other immediately before the nip portion. At this time, if a distance between the intermediate transferring belt and the sheet becomes so small because of deformation of the intermediate transferring belt, the sheet may rub a toner image on the belt before it enters the nip portion, and a part of the toner image stuck to the belt may be dispersed on the sheet. In particular, since a rear end portion of a sheet can be freely moved after the rear end of the sheet goes out from the nip portion of the pair of registration rollers, the intermediate transferring belt is likely to come in contact with the sheet if the sheet slightly hops when the flexure occurs in the intermediate transferring belt. In this case, even if a toner image is correctly formed, a quality of an image outputted after the secondary transfer is negatively affected.